A typical field effect transistor (MOSFET) is used nowadays instead of a bipolar transistor at high frequencies (1-2) GHz in cellular and personal communication services (PCS) wireless communication amplifiers.
One reason for the preference for a MOSFET device over a bipolar device is that a MOSFET device is a more linear device, that is it has a less non-linear transfer characteristic, than a bipolar device.
Indeed, each P-N junction has a non-linear transfer characteristic. A typical bipolar device has two P-N junctions and is an inherently non-linear device with an exponential transfer characteristic.
On the other hand, a typical MOSFET transistor, though it nominally has also two P-N junctions, can be modelled by a resistor whose conductivity is modulated by changes in gate voltage. It follows, that a MOSFET transistor is in a sense a less non-linear device than a bipolar transistor.
A typical MOSFET transistor has a gate-voltage-to-drain-current transfer characteristic that can be approximated by a parabolic function at small gate voltages and by a linear function at intermediate gate voltages. At large gate voltages, the transfer characteristic of a typical MOSFET device becomes saturated.
The high-efficiency power amplifiers used in cellular and PCS infrastructure are biased class AB or B. This means that the DC bias point is located in the low current portion of the transfer curve. Therefore, most of the distortion produced by the device comes from operating the device in the low current portion area of the transfer characteristic.
What is needed is an RF MOSFET device with a reduced parabolic and an extended linear portions of the transconductance curve at low currents so that the RF MOSFET device has less intermodulation and crosstalk distortion when it is used in a power amplifier for wireless communications.